1. Field of the Invention
This invention relates generally to a vehicle stability enhancement system and, more particularly, to a vehicle stability enhancement system that employs a driver workload estimator for estimating the driver workload.
2. Discussion of the Related Art
Modern vehicles sometimes incorporate active vehicle control systems. One such system is known as a vehicle stability enhancement (VSE) system that assists the vehicle operator in providing vehicle handling on surfaces such as wet or uneven pavement, ice, snow or gravel. The VSE system typically senses wheel skid based on inputs from a wheel speed sensor, a steering angle sensor, a vehicle speed sensor and a vehicle yaw rate sensor. The VSE system uses these inputs to reduce engine torque and apply differential braking to help maintain the vehicle travel along the intended path.
Additionally, active chassis control systems have been emerging in the art that are used in combination with VSE systems. The chassis control systems typically include differential braking control, real-time suspension damping, rear-wheel steering and active front steering control. With the capability of controlling chassis dynamics in real time, the active chassis control systems can be used to enhance the vehicle handling performance.
Another active vehicle control system is known as an active front steering (AFS) system that provides automatic front-wheel steering. AFS systems typically employ a steering actuator system that receives an operator intended steering signal from a hand-wheel sensor, a vehicle speed signal and a vehicle yaw rate signal, and provides a correction to the operator steering signal to cause the vehicle to more closely follow the vehicle operator's intended steering path to increase vehicle stability and handling. The AFS system is able to provide steering corrections much quicker than the vehicle operator's reaction time, so that the amount of operator steering is reduced. The AFS system provides a more direct vehicle steering under normal road conditions at low and medium speeds, reducing operator steering effort. The AFS system also may help to increase vehicle agility in city traffic or during parking maneuvers. The AFS system also provides less direct vehicle steering at higher vehicle speeds.
Vehicle stability enhancement systems measure the driver's desire of vehicle maneuvers to control inputs, such as steering wheel, brake pedal, etc., and compare the vehicle response with the desired maneuver. The design of a vehicle stability enhancement system usually includes a command interpreter to determine the driver's desire, and a feedback control for vehicle yaw-rate and lateral dynamics to control the vehicle response to meet the driver's intended performance.
The known vehicle stability enhancement systems do not consider the driver's physical and mental status, and cannot identify certain situations when a driver needs additional help. A better design for a vehicle stability enhancement system may fulfill the driver's desire of performance driving. When the level of the driver's workload is recognized, the information of the workload can be used to make further adjustments in the command interpreter and the feedback control.